wasi-common error cleanup: part 1, yanix (#1226)

* Reuse std::io::Error for raw *nix errno

This commit removes custom `yanix::Errno` and instead (as was
previously suggested) reuses `std::io::Error` to generate and wrap
raw *nix errno value.

* Update wasi-common to use new Yanix error type

This commit updates `wasi-common` to use new way of handling raw
OS error in `yanix`; i.e., via re-use of `std::io::Error` instead
of a custom `Errno` enum.

* Fix formatting

* Unwrap if io::Error created from raw OS error

This commit calls `unwrap` on `err` if that one was created via
`io::Error::last_os_error()`. It also refactors error matching
in several syscalls on the BSD platform (mainly).

crates/wasi-common/yanix/src/socket.rs

@@ -1,4 +1,4 @@
-use crate::{Errno, Result};
+use crate::{Error, Result};
 use std::os::unix::prelude::*;
 
 #[derive(Debug, Clone, Copy)]
@@ -15,7 +15,7 @@ pub unsafe fn get_socket_type(fd: RawFd) -> Result<SockType> {
     use std::mem::{self, MaybeUninit};
     let mut buffer = MaybeUninit::<SockType>::zeroed().assume_init();
     let mut out_len = mem::size_of::<SockType>() as libc::socklen_t;
-    Errno::from_success_code(libc::getsockopt(
+    Error::from_success_code(libc::getsockopt(
         fd,
         libc::SOL_SOCKET,
         libc::SO_TYPE,